When a viscous liquid, such as drilling fluid, is pumped under high pressure conditions, various simultaneously occurring detrimental conditions cause failure of valves and valve seats. In some cases, metal parts of valves and valve seats become excessively worn either by the cutting, abrasive activity of high pressure liquid containing abrasive constituents. It is not unusual for a valve and valve seat to become eroded by high pressure drilling fluid in a relatively short period of time when seal leakage occurs during pumping operations. During each pumping stroke and suction stroke, extremely high velocity flow conditions occur between the sealing element and seat of the valve mechanism. This high velocity condition, coupled with the abrasive character of the fluid being pumped causes severe erosion conditions within a relatively short period of time.
Another condition which is detrimental to the service life of pump valves is the mechanical shock which is developed as the valve is shifted to its closed position under the influence of spring force and pressure induced force. At times, this mechanical shock can be sufficiently severe to induce metal failure such as by breakage of valve guide stems and other valve parts. Metal failure also occurs when a condition of peening develops due to the mechanical shock of valve closure. Excessively worn valve guides can cause valve stem breakage and can also cause accelerated wear of the valves and valve seats of drilling fluid pumps and other pumps of similar nature.
The problem of seal wear in pump valves is accelerated by a condition known as seal extrusion. Typical pump valves for drilling fluid pumps incorporate a metal valve body having a metal seal retainer ring which is secured by threads to the valve body. The valve body together with the seal retainer ring form a circular seal receptacle within which is retained a circular sealing member composed of elastomeric material. Typically, the sealing ring is replaceable. The retainer ring is provided with hammer lugs, enabling the retainer ring to be driven into tight assembly with the valve body to secure the sealing member in assembly. Conversely, the hammer lugs of the retainer ring permit it to be loosened to thus permit replacement of worn seal rings.
The seal rings of drilling fluid pump valves such as those manufactured by Mission Manufacturing Company of Houston, Tex., typically form a tapered or frusto-conical sealing surface which is slightly offset in comparison to a tapered, frusto-conical sealing surface defined by the valve body. The tapered sealing surface of the seal member is adapted for sealing engagement with a correspondingly tapered frusto-conical seat surface defined by a seat insert with which the pump mechanism is provided. As the valve assembly is seated against the seat surface of the seat insert by a combination of forces including the spring force of the seat assembly and pressure induced forces acting upon the valve assembly, the high pressure condition to which the valve assembly is subjected acts upon the elastomeric sealing element, attempting to force a portion of the sealing element into the narrow space between the valve and valve seat as the valve moves to its fully seated relationship with respect to the seat. This pressure extrusion causes severe tensile stressing of the elastomeric material at the peripheral interface between the sealing element and the metal support surface of the valve body.
Pressure induced deterioration of the elastomer component of pump valves is largely caused by hysteresis. In drilling fluid pumps the valves must operate at from 50 to 150 cycles oer minute. Following a compression or pumping stroke of a positive displacement piston the pump valves operate as check valves, closing under the influence of back pressure as the pistons reverse their reciprocity movement. The back pressure acting on the pistons is severe, causing them to slam to their closed positions. As the valves close the back pressure causes compression throughout the elastomer material of the pump valve. If any voids exist when the valve is seated against the valve seat the elastomer material is pressure induced to deform and thus substantially fill the void. For the most part, such voids will inevitably exist. The elastomeric material will therefore be deformed from 50 to 150 cycles per minute to fill the voids. Though the voids are typically small, being annular crevices, etc., the elastomer deformation necessary to fill the voids, being effective in a limited portion of the seal, will cause severe tensile stress to be placed on limited portions of the sealing element. The hysteresis effect due to cyclic stressing of the elastomeric material will result in material failure in the vicinity of the voids being filled. As elastomer failure occurs, the elastomer material sloughs or is torn away thereby developing an even larger void which must be filled on subsequent closure of the valve. This activity places even greater tensile stress on the remaining elastomeric material, thus causing it to deteriorate further by the hysteresis induced sloughing or tearing activity. Soon sufficient elastomeric sealing material is lost that the sealing capability of the valve becomes impaired, resulting in the necessity for pump shutdown and repair.
It is, of course, desirable to provide a valve and valve seat assembly for high pressure pumps such as drilling fluid pumps which will remain serviceable for extended periods of time to thus minimize repair costs and minimize down time of the pumping mechanism.